Much effort has been spent in improving methods used to remotely characterize media, which can be referred to as media identification. In general, media identification is attained using electromagnetic energy. wherein radar is used accordingly. In particular, high resolution radar is a preferred means of performing media identification which may include the detection and identification of subsurface mines as well as be applied to measure moisture contents in media.
Carrier signals presently used in radar systems are sinusoidal. These carriers can be coded or non-coded depending on the type of radar used. Codes used in high resolution radar typically use Barker, Frank, Costas and Welti based codes. Generally, carriers of ground penetrating radars are not coded. Moreover, most ground penetrating radars are non-portable and not packagable in a hand-held radar unit for subsurface surveys. In most current ground penetrating radars, the antenna is designed to slide on the surface of the medium being tested. This action inherently limits their use. Another method for this intended application is the use of synthetic aperture radar which is a high resolution radar. This type of radar system takes advantage of the forward motion of an airborne radar system for producing an equivalent antenna array that may be thousands of feet long. Moreover, the beam width of such this equivalent array is roughly half that of a real array of the same length. The outputs of the array are synthesized in a signal processor from the returns received by the real radar antenna over period of up to several seconds or more.
Prior art teachings of apparatus intended for subsurface media identification is U.S. Pat. No. 4,937,580 entitled "Geophysical Radar Apparatus and Method" by Wills. Another prior teaching of apparatus intended for subsurface media identification using physical and electrical properties of a dielectric object using sequential spatial and spectral microwave data is U.S. Pat. No. 5,327,139 entitled "ID Microwave Holographic Sensor" by Johnson. Both of these prior art teachings do not use the current invention's efficient profile inversion methodology for identifying subsurface objects that requires less power usage.
Media identification generally occurs in three parts: detection, discrimination, and recognition. Feature extraction from the reflected electromagnetic energy is performed in the latter for three reasons (1) to optimize recognition system performance, (2) to reduce the amount of information to be processed, and (3) to ensure robustness or invariance of the recognition system. For example, a set of features may actually be composed of the fast Fourier transform of the reflected energy, target's radar cross section (RCS) and the permittivity, velocity of propagation, permeability, susceptibility, and conductivity profiles of the medium in which the target exists. While the target's RCS is produced through the application of various prior radar methods, application of inverse scattering theory is utilized to construct the mentioned profiles. Electromagnetic target identification has numerous practical applications in subsurface/ground-penetrating radar, geophysical sensing and nondestructive testing. The portable high resolution radar emits an incident wave and observes a reflected wave to detect the electromagnetic properties and presence of objects. Also, in the operations of nondestructive testing of media and remote sensing, it is desired to apply target or pattern recognition for the purpose of identifying objects, defects or any other kind of targets and discriminate between them and clutter. In this case the electromagnetic profiles mentioned above are considered important parts of the recognition process. These profiles are used in the discrimination and identification stages. From a physical point of view, different bodies, media or materials possess different profiles of electromagnetic properties as seen from models of such properties. For example, a charge model for permittivity of a dispersive medium has an equation of motion for the charge bounded by harmonic force and acted on by an electric field and where the effects of magnetic forces can be neglected. The permittivity of this dispersive media is described by a function of i) the number of the molecules per unit volume, ii) the number of the electrons per molecule and iii) a damping constant. Thus, the permittivity profile of every material is unique. Accordingly, media recognition processes that utilize reflection, conductivity, permittivity and permeability profiles provide a reliable way for media identification. Improvement in the speed by which the results are generated through this process and their accuracy is logically vital for any military mission such as the automatic detection and identification of subsurface mines. A portable high resolution radar can be used to generate electromagnetic profiles which are utilized for automatic recognition processes. High resolution radar can be achieve by transmitting a low-peak-power, coded pulse of long duration and then compressing on reception. A radar system that incorporates pulse compression processing provides improvement in the detection performance, reduction in the mutual interference and an increase in the system operational flexibility. The instant invention satisfies these requirements by providing a portable hand-held, relatively low power consuming, processor based high resolution radar system for generating electromagnetic profile of a medium and subsurface object (s) contained therein.